It is well known in the art to provide missiles with receivers so that transmitted guidance signals can be reliably received, which guidance signals can serve to guide the missile to intercept a selected target.
One such missile system of particular interest to the present invention involves the transmission of laser guidance signals, which are received by one or more aft looking receivers located on the rear of the missile to be guided. By this arrangement, precise guidance information can be readily provided to the missile, such that it can be guided to intercept an enemy aircraft or missile, with this being accomplished in a way that make effective enemy countermeasures difficult to achieve.
In this missile system of interest, a detector used by the missile receiver needed to be placed comparatively close to the motor of the missile, but unfortunately, it was then found that the plume of radiant energy emanating from the motor gave rise to noise that overshadowed the guidance signals sought to be received by the detector.
One originally considered solution to the problem of removing the motor plume from the field of view of the detector involved the use of a metal shield protruding along the missile axis between the detector and the exhaust nozzle. However, in order for such a shield to be able to accomplish its purpose, it by necessity had to protrude several inches beyond the limiting envelope of the missile, and this became quite unacceptable from the aerodynamic standpoint.
Certain optical solutions to the problem posed by the motor plume were also proposed, but it was found that a conventional lens system had a focal length to diameter ratio as to make it impractial, as would greatly diminish the level of guidance signals that could be received at the detector. Consequently, it became apparent that the best approach involved the use of an optical wedge arrangement of a configuration such that undesirable radiant energy could be successfully blocked from reaching the detector.
In accordance with one aspect of this invention, we utilize an optical window formed from two wedges with an air gap therebetween. In that way, energy from within a desired field of view can pass through such window, while unwanted energy, coming in from a different angle, strikes the internal surfaces between the wedges, such that the critical angle is exceeded. This energy may, for example, emanate from engine exhaust, and it will be totally reflected internally, and absorbed by the outer walls of the window.
A number of prior art devices have utilized frustrated internal reflection filters or couplers, and one such device teaches the use of a pair of wedges having a gap between the angled surfaces, which gap may be electrically varied. This functions to open and close the air gap, and can therefore be used as a modulator to alternately pass or cut off radiation passing through the unit. Other teachings involve the input member being made of a different material than the output member, thus to accomplish a type of filtering.
Still another prior art device teaches the use of a wedge shaped spectral filter utilized to separate two different frequency signals by reflecting one signal from the top slanted surface and the other signal from the bottom slanted surface.
The approach we utilized to accomplish the goals of this invention differs significantly from these earlier approaches.